Bioluminescence is the emission of visible light by living organisms. It is a characteristic of many terrestrial organisms eg. the firefly, glow worm, certain click beetles and near-surface ocean creatures too. It is in the deepest parts of the oceans, however, that bioluminescence has reached a high state of development; more than two thirds of the species there produce light. Some fishes, squids, and euphausiids (krill) possess "search lights" with lenses, reflectors, and irises almost as complex as those of the human eye.
Its most common occurrence to the sailor is in the often brilliantly luminescent bow wave
of a surface ship. The organisms responsible are almost
always dinoflagellates, single-cell algae. These organisms are mechanically excited by the ship'spassing and consequently emit light. This can also be brought about by dolphins and smaller fish or humans swimming.